1. Field of the invention
This invention relates to a dosing or metering apparatus which supplies bulk materials to, for instance, a blender, in a controlled manner, either in portions or continuously. The bulk materials to be handled can be free-flowing or non-free-flowing like pellets, granulates, powders, regrinds, flakes, flours, farinas, etc. This kind of dosing apparatus can be part of a so-called blending station, where more than one component (ingredient) is to be mixed into a specified blend. But it can also be used as a single supplying/monitoring unit wherever the controlled flow of a bulk material is required. Major fields of application are the plastics processing industry, e.g. extrusion and injection molding plants, and feed mills and food processing plants.
2. Description of the Prior Art
Well established in the plastics processing industry are, for example, blending stations consisting of a central mixing hopper with up to six dosing units sitting on top of it. Such a station is able to produce blends out of up to six components, according to defined formulas. The working principle of these dosing units (also called metering units) is strictly volumetric. The metering elements of these units can be, for instance, rotating disks with holes, star feeders, or augers. The monitoring of the rotation of these elements is achieved either by counting the revolutions or fractions of them, or by time control. The amount of bulk material metered in a given period of time is the product of volume unit per increment, e.g. disk holes multiplied by the number of increments. While this dosing method is quite simple, it is not very reliable and there is no assurance of accuracy. One can never be certain that the metering elements, e.g. holes in the metering disks, are uniformly filled or even if there is any material in them to begin with. Furthermore, quite a number of bulk materials do not lend themselves to this dosing procedure, due to certain physical properties. Since no confirmation is being generated, there is no reliable verification that all the components of a blend are being metered properly and therefore present at the required ratio.
Of course, there has always been the possibility of using gravimetric methods in order to circumvent the above mentioned shortfalls. Working in the so-called lost weight mode, one can make use of various kinds of weigh scales such as weighing hoppers on load cells. All of these scales are expensive, bulky and often their accuracy does not suffice. Furthermore, none of these weighing systems lend themselves to the construction of a gravimetric dosing apparatus, which could replace certain volumetric dosing units.